Death is a funny thing. Death is almost like a dream, where you can walk anywhere you will, but no one can see you; no one can hear you; no one can touch you - no one but the other dreamers. Everything shimmers, and the color seems to be leached away from the world, but the world is still there - so close you can nearly touch it, but you always seem just a millimeter away from everything. Almost as though the world is four dimensional rather than three dimensional, but you are still trapped in a three dimensional plane, a millimeter from the ones you love no matter what you do.
Then, there is the in between. Where people are separated into this world and the next; where part of them stays in our plane, and part of them is in the plane of death, forcing them to exist in both - and neither - places at once. They shuffle in and out of existence, only a rippling shadow of what they were before. They describe it as a prison shackle; the side of them that is with the living has all the control, while the side on the world of the dead is shackled to it. They say that only the best parts of you die, leaving the darkest, blackest parts of your soul in the land of the living.
The dark shadow of yourself controls you, dragging you along for the ride as it goes on a rampage of desire and anger and blackness. You can't do anything but watch as your shadow self torments people in the land of the living, stripping away their happiness, their hope, their warmth, their souls. You can scream and scream to try and warn people to get away from your dark self, but they can't hear you - you're a millimeter away from them, but can never warn them. Never touch them. But your shadow self can.
People hate you. They fear you. They watch you with angry, terrified eyes, and you watch them as their eyes go cold and empty. Your dark self continues seeking out what it used to have - lightness, happiness, and everything it has lost - by taking it from others. By sucking away their soul. And you can't help but hate that you are trapped this way, half way in the land of the living and half way dead; you cannot rest in peace. You grow angry and you go mad, wishing only that this hadn't happened to you - wishing only that it hadn't made you into a Dementor. But the Demontor is only what was already inside you - it is you.
This is what the Veil does.
It corrupts, it destroys, it shreds your soul in two. The voices from the veil are the souls who have gone mad watching their shadow selves destroy innocent people. The living who see the Veil cannot help but feel a sickness inside them, as though they know already what evil corruption it wreaks on your spirit.
If only Sirius Black had known before he had fallen in. If only he had known, he would have killed himself to avoid this fate - but he had done it for Harry. He'd do anything for Harry. Harry was his everything.
Nothing hurt more than watching his shadow self stalk the streets during the war, searching for Harry, searching to destroy Harry.
If only Sirius Black had a way out, but he had no way out. He was trapped. He could feel his mind starting to slip away from him, he could not find a way to force his shadow self to obey him. But he couldn't. For two years, he watched ice spread beneath his black cloak, he watched people scream, he watched their hope leave them - all because they saw him.
Just when he thought he could not bear it anymore, there was a ripping noise, and suddenly he was free from his shadow self. He was standing by himself in a clearing, and others materialized around him - other ghosts, the ghosts of his loved ones. James, Lily, Remus... His heart was soaring.
And most importantly, in the middle of the clearing stood his godson, Harry, with the resurrection stone in his hand - the resurrection stone had separated him from his shadow self. He was finally free.
Harry saved him.